1. Field of the Invention
The present invention relates to an image-forming apparatus.
2. Description of the Related Art
A conventional image-forming apparatus such as a copier or a laser beam printer uses an intermediate transfer member (referred to hereafter as an intermediate transfer belt). During a primary transfer process performed in this type of image-forming apparatus, a toner image formed on a surface of a drum-shaped electrophotographic photoreceptor (referred to hereafter as a photosensitive drum) is transferred onto the intermediate transfer belt by supplying a voltage from a high voltage power supply to a primary transfer member disposed in a position opposing the photosensitive drum. By executing this primary transfer process repeatedly in relation to toner images in a plurality of colors, toner images in a plurality of colors are formed on the surface of the intermediate transfer belt. Next, as a secondary transfer process, a voltage is supplied from the high voltage power supply to a secondary transfer member such that the toner images in the plurality of colors formed on the intermediate transfer belt are transferred together onto a surface of a recording material such as a sheet of paper. The toner images transferred together onto the recording material are then permanently fixed by fixing means, whereby a color image is formed.
Japanese Patent Application Publication No. 2012-98709 discloses a configuration in which primary transfer of a toner image onto an intermediate transfer belt is performed by applying a voltage to a current supply member that contacts the intermediate transfer belt such that a current flows from the current supply member to a plurality of photosensitive drums via the intermediate transfer belt. With this configuration, primary transfer can be performed without a high voltage power supply used exclusively for primary transfer, and as a result, reductions in the cost and size of the image-forming apparatus can be achieved.
Further, with the configuration disclosed in Japanese Patent Application Publication No. 2012-98709, a surface potential (referred to hereafter as a belt potential) of the intermediate transfer belt is kept constant by connecting a constant voltage element to respective support rollers, and in so doing, a primary transfer performance can be stabilized. An example in which a Zener diode is used as the constant voltage element is disclosed. By connecting a Zener diode to the respective support rollers in this manner, the belt potential is prevented from increasing to or above a potential generated in the Zener diode, and as a result, the belt potential of the intermediate transfer belt, which contacts the respective support rollers, can be kept constant.
However, with the configuration disclosed in Japanese Patent Application Publication No. 2012-98709, according to which primary transfer is performed from the plurality of photosensitive drums by causing a current to flow in a circumferential direction of the intermediate transfer belt, it is difficult to maintain the belt potential at an appropriate value in both an image-forming station near the current supply member and an image-forming station far from the current supply member. When the belt potential cannot be maintained, a required toner amount cannot be transferred onto the intermediate transfer belt, and as a result, problems such as density reduction occur.
More specifically, in a configuration where primary transfer is performed by causing a current to flow in the circumferential direction of the intermediate transfer belt, a voltage drop occurs in the belt potential in each image-forming station due to resistance in the circumferential direction of the intermediate transfer belt. As a result, the belt potential decreases steadily toward the image-forming stations further from the current supply member, possibly leading to a large potential difference between image-forming stations on an upstream side and a downstream side of a movement direction of the intermediate transfer belt. In this case, an appropriate belt potential for primary transfer cannot be formed in the respective image-forming stations, and therefore primary transfer cannot be achieved favorably.